The repetition of the r sound in "heaR", "Rushing", "RoaRing", and "shRill" suggests the image and sound of the train rushing past. When something fast moves by you, there's a rush of air that flies by, and this sound and movement of air, mixed with the grinding of the train on the tracks is meant to be represented by the 'r' sound, based on the consonance in the line.
